The present invention relates generally to chemical technology, and more particularly, to improved methods for carrying out chemical reactions to yield desirable products.
Much of the chemical industry is based on chemical reactions which convert raw materials into basic chemical compounds. These compounds are in turn converted to end products which are sold to other industries or directly to consumers. As an example, acrylonitrile can be prepared by catalytically reacting propylene, oxygen, and ammonia. The product can in turn be used to prepare acrylic-based materials--e.g., acrylic fibers, acrylonitrile-butadiene-styrene (ABS) resins, or a host of other important polymers.
On a commercial level, chemical products often have to be produced in very large quantities for the sake of efficiency--upwards of 1 million pounds per day. This scale of manufacturing requires large chemical plants with many individual unit operations for carrying out one or more chemical reactions, each of which may include a multitude of reaction steps. Land size requirements for accommodating such a plant are also considerable.
Batch processes for preparing basic chemicals are limited by the size of the reactor vessel, as well as the time required for reagents to be sufficiently mixed. Continuous processes are also limited in terms of production capacity by a variety of factors, such as the "throughput" rate, which dictates how quickly reagents can be fed to a reactor, heated, and sufficiently reacted to yield a product. Other factors also affect throughput rate, e.g., how quickly the product can be isolated and taken out of the reaction system.
If the product is an intermediate used to form a final product, an additional reactor may be needed at another site. Considerable engineering effort may be required to transfer products and other reagents to the second site. The added time requirements in this effort may further limit production capacity.
An example of a continuous reaction system is the fluidized bed reactor, which can be very useful because it allows good mixing of reagents and high product yield. Moreover, these reactors are usually very efficient, due in part to the fact that the catalyst can be removed from the bed and regenerated without shutting down the unit. Hydrogen cyanide is sometimes produced on a commercial scale by this technique. In that instance, ammonia and air are catalytically reacted with methane in the fluid bed to yield water and the desired product.
While systems like fluidized bed reactors exhibit many advantages, their use has some drawbacks as well. For example, the ammonia-methane reaction can be difficult to control because of non-uniformity in the bed itself. Some of this non-uniformity appears to be due to irregular flow of the reagents through the feed nozzles. "Hot spots" in the bed may develop, due to uncontrollable temperature excursions. These hot spots can cause coking in the reactor and ultimately, serious damage to the bed. Destruction of the fluidized bed when it contains expensive catalysts like platinum can in turn be a serious economic problem.
It is clear from the foregoing, as well as from a survey of the state of the art, that new processes for carrying out chemical reactions would be of considerable interest in the industry. These processes should be capable of handling large volumes of reagents quickly and efficiently, with high product yield. To that end, the processes should permit the use of high temperatures and high pressures when necessary. The processes should also accommodate more than one separate reaction, when multiple reactions are required to obtain a final product. Moreover, the processes should minimize plant size requirements as much as possible. Still another attribute of such processes would be the utilization of excess energy resulting from at least one of the reactions, e.g., heat energy which might otherwise be wasted.